During the past several years, the popularity and viability of fuel cells for producing large and small amounts of electricity has increased significantly. Fuel cells conduct an electrochemical reaction with chemicals such as hydrogen and oxygen to produce electricity and heat. Some fuel cells are similar to batteries but can be “recharged” while still providing power. Fuel cells are also much cooler and cleaner than electric generators that burn hydrocarbons.
Fuel cells provide a DC (direct current) voltage that may be used to power motors, lights, computers, or any number of electrical appliances. There are several different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by the type of electrolyte used. Fuel cell types are generally categorized into one of five groups: proton exchange membrane (PEM) fuel cells, alkaline fuel cells (AFC), phosphoric-acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and molten carbonate fuel cells (MCFC).
Each of the fuel cells mentioned above uses oxygen and hydrogen to produce electricity. Ambient air typically supplies the oxygen for a fuel cell. In fact, for the PEM fuel cell, ordinary air may be pumped directly into the cathode. However, hydrogen is not as readily available as oxygen. Hydrogen is difficult to generate, store, and distribute for a number of reasons including high flammability. Thus, strict safety precautions must be taken in order to reduce potential hazards.
One common method for producing hydrogen for fuel cells is through the use of a reformer. A reformer is fed hydrocarbons or other fuels that produce hydrogen. The hydrogen produced by the reformer can then be fed to the fuel cell where that hydrogen reacts with oxygen or another oxidant to produce the desired electricity. The use of a reformer allows for the production of electricity using propane, butane, or a number of other readily accessible natural gases as the hydrogen fuel source.
These natural gasses are typically stored in a container at a high vapor pressure (greater than 1 atmosphere partial pressure) or low vapor pressure (less than 1 atmosphere partial pressure) and are accessed when hydrogen is required by the system. While the storage of the hydrocarbons has traditionally been fairly simple, it has traditionally been difficult to predict the level of fuel remaining in a pressurized container.
One previous method for determining the remaining amount of fuel in a pressurized container involved estimating the amount of fuel that has passed from the pressurized container into the reformer. The estimated amount of fuel is then subtracted from the container capacity to determine the likely amount of remaining fuel and subsequently, the amount of time an automobile or other device incorporating the fuel cell could function before refueling. While this method was generally useful, it was expensive to implement and was not sufficiently accurate for sources requiring precise fuel level information.
An additional previous method for determining the remaining amount of fuel in a pressurized container involved routing both fuel and pressurized air to a number of pressure sensors to establish a differential pressure signal. The pressure difference between the fuel and the pressurized air could be used to predict low fuel conditions. However, in order to supply the fuel and the pressurized air to the pressure sensors, at least two fluid interconnects had to be routed to the fuel supply. These fluid interconnects significantly increased the cost of the fuel supply containers and increased the likelihood of fuel leaks due to an insufficient seal on one or more of the interconnects.